The concentration dependence of light scattering of three proteins, ovalbumin, ribonuclease A, and ovomucoid, and binary mixtures of the three proteins were measured at various compositions and pH values at total protein concentrations of up to 35 g/l, under conditions such that ribonuclease bears either a net positive or negative charge and the other proteins bear a net negative charge. The concentration dependence was interpreted in the context of three models: a statistical thermodynamic model, an extension of the effective hard particle allowing for the presence of non-additive interactions between species in a binary mixture, and, most recently, introduction of a new model in which interactions between protein molecules are described in terms of a square well potential.